from the u.s. postal service. because it's nice to have an extra pair of hands around for the holidays. but the acidic levels in some foods can cause acid erosion. the enamel starts to wear down, and you can't grow your enamel back. my dentist recommended that i use pronamel, because it helps to strengthen the enamel. and i believe it's doing a good job. >>> one staff member of sandy hook elementary school actually encountered the gunman. school nurse sally cox explained the incident on "60 minutes." >> all of a sudden, i heard a very loud popping noise. i mean, a noise that i've never, ever heard before. and my first thought was, was this something with the heating system, or did something fall. and i called out to the secretary, barb, what is that? she called out to me by name, she said, "sally" -- and i could hear fear in her voice. >> it was something about the way barb called out your name? >> yes, yes. she just had this horrible sound of fear in her voice. and i just knew it was -- sounded like a bad thing b